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    TD_RAWREAD/ETD_RAWREAD -- read raw data from the disk.

    These commands read a track of raw data from disk and deposits it in
    the provided buffer. The data is taken straight from the disk with
    no processing done on it. It will appear exactly as the bits come out
    off the disk, hopefully in some legal MFM format.

    This interface is intended for sophisticated programmers only.
    Commodore-Amiga reserves the right to make enhancements to the disk
    format in the future. We will provide compatibility via the
    CMD_READ and ETD_READ commands, anyone using TD_RAWREAD is
    bypassing this upwards compatibility, and may thus stop working.

    io_Device       preset by the call to OpenDevice()
    io_Unit         preset by the call to OpenDevice()
    io_Command      TD_RAWREAD or ETD_RAWREAD.
    io_Flags        if the IOTDB_INDEXSYNC bit is set then the driver
                    will make a best effort attempt to start reading
                    from the index mark.  Note that there will be at
                    least some delay, and perhaps a great deal of delay
                    (for example if interrupts have been disabled).
    io_Length       Length of buffer in bytes, with a maximum of 32768
    io_Data         Pointer to CHIP memory buffer where raw track data is
                    to be deposited.
    io_Offset       The number of the track to read in.
    iotd_Count      (ETD_RAWREAD only) maximum allowable change counter

    io_Error - 0 for success, or an error code as defined in

    The track buffer provided MUST be in CHIP memory

    There is a delay between the index pulse and the start of bits
    coming in from the drive (e.g. dma started). This delay
    is in the range of 135-200 microseconds. This delay breaks
    down as follows: 55 microsecs is software interrupt overhead
    (this is the time from interrupt to the write of the DSKLEN
    register). 66 microsecs is one horizontal line delay (remember
    that disk IO is synchronized with agnus' display fetches).
    The last variable (0-65 microsecs) is an additional scan line
    since DSKLEN is poked anywhere in the horizontal line. This leaves
    15 microsecs unaccounted for...  Sigh.

    In short, You will almost never get bits within the first 135
    microseconds of the index pulse, and may not get it until 200
    microseconds. At 4 microsecs/bit, this works out to be between
    4 and 7 bytes of user data of delay.

    This command does not work reliably under versions of Kickstart
    earlier than V36, especially on systems with 1 floppy drive.


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